Your amp going into protect has nothing to do with power connections. So stop looking there.

The protect circut protects the speaker outputs. You are having issues on the speaker end.

This is typically due to a speaker / amp mismatch. or a blown sub.
You need to know what ohm load your sub runs at. Single 1, 2, or 4 ohms.
Dual 1, 2, or 4 ohms.

Then you need to know what ohm load the amp is stable at. Most amps say if they are 2 ohm stable will go into protect if connected to a sub wired at 1 ohm. Competition amps are better at handleing reduced output loads but even there many will go into protect if pushed hard enough.

My gut feeling is that your 6 channel is no more than 2 ohm stable and your sub is wired to a 1 ohm load. Hence the amp goes into protect. You use a ohm meter and measure across your speaker leads while disconnect from the amp. THis will verify things.

Yeah boyyyeeeeee wat he said

90% of the time if its a short on your powering side you will blow your fuse (no power likes a dead short), if it was a loose ground it will still work but you will end up with only hot connections and in the end a blown fuse or melted terminations on your amp, protection mode is only for the speaker outputs, if they are not connected that could be why or if the is a small trace of wire across the terminals , likewise your ohm values need to be correct as our mate above said